\font\fontA="[Iwona-Regular.otf]"
\font\fontB="[Iwona-Regular.otf]:mapping=tex-text"

\uselanguage{french}

\newdimen\savehsize
\savehsize\hsize

\def\test#1#2{{\tt#1}\endgraf\hsize=3pt\noindent #2\endgraf\hsize=\savehsize}

\def\testA#1{\test{#1}{a l'information}}
\def\testB#1{\test{#1}{l'information}}

\fontA

\testA{Without lccode}

\bgroup
\lccode"27="27
\testA{With lccode 27->27}
\egroup


\fontB

\testA{Without lccode (with tex-text)}

\bgroup
\lccode"2019="0027
\testA{With lccode 2019->27 (with tex-text)}
\egroup

\bgroup
\lccode"2019="2019
\testA{With lccode 2019->2019 (with tex-text)}

\testB{the same (why is there no break?) ...}
\egroup


\bye
